Ethanol is an important source of energy and useful as an alternative to petroleum based gasoline and diesel products. Ethanol is produced by fermentation of a wide variety of organic feedstocks to provide a beer that is distilled and dehydrated to produce a high purity product. Aden, et al, in “Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover”, NREL Report No. TP-510-32438; and Madson, P. W. and D. A. Monceaux, (Fuel Ethanol Production), in “Fuel Alcohol Textbook” Alltech Inc., 1999; provide an overview of the process. Substantial capital investment and energy are required to concentrate ethanol from a beer concentration of about 6 to about 16 wt % ethanol in water to a minimum of 99.5 wt % ethanol used in fuel grade ethanol. The concentration process operates through the ethanol-water azeotrope of about 95 wt % ethanol that has to be further dehydrated. In a typical fuel ethanol plant, the ethanol is first distilled from beer in a column, known as the beer column, designed to recover almost all ethanol from the spent beer. Vapor from this column passes to a second column, the rectifier column, wherein ethanol is concentrated to about 92 wt % ethanol, the substantial portion of the remainder being water. The vapor from the rectifier column is further dehydrated by passing it through a bed of a dehydration agent to provide the required 99.5 wt % ethanol vapor stream. Typically the dehydration agent is a 3 A molecular sieve material, however other dehydration agents such as corn grits, for instance, can be used. When the bed of dehydration agent is spent, it is taken off line and regenerated to provide an active dehydration bed and an ethanol-water vapor or liquid stream that is typically recycled to the rectifier column.
This latter dehydration process has attracted much attention of scientists and engineers because it is a critical and expensive step in the refining process. Typical plants now use 3 A molecular sieve beds as the dehydration agent. Molecular sieves adsorb water, in this case from an ethanol vapor, and must be regenerated as the sieve becomes saturated. One way to do this is to pass very hot, dry gas through the bed. The fuel ethanol industry found that under the conditions of ethanol dehydration, the hot gas caused the molecular sieves to crumble (Madson, 1999). Instead, they began to regenerate the bed with 99.5 wt % ethanol vapor available at the desired temperature from a parallel molecular sieve bed. They also learned to retain the heat of adsorption in the bed (Garg, D. R. and J. P. Ausikaitis, “Molecular Sieve Dehydration Cycle for High Water Content Streams”, CEP, April 1983, p 60-65) so that the countercurrent flow of ethanol vapor could regenerate the bed with little or no heat addition (Aden, 2002). But a problem created by this approach is that the ethanol vapor used in regeneration is contaminated with water. This ethanol must be returned to the rectification section of the distillation equipment and redistilled to 92 wt % concentration. Thus, it is accepted, and most plants are designed for the fact, that a certain fraction of the capacity of the rectifier column will be taken by the regenerate stream of ethanol-water provided by the regeneration of the molecular sieve or other dehydration beds. However, because of the significant emphasis on the dehydration process, alternative processes have been developed to dehydrate the ethanol vapor stream that require less volumes to be recycled to the rectifier.
Brazilian patent, BR 8703445(A1), describes an alternative process for regenerating molecular sieve beds, wherein a hot inert gas such as carbon dioxide, is passed through the molecular sieve bed. Such a process is one example of a variety of processes that may have the potential to decrease the amount of ethanol being recycled to the rectifier column as a result of the dehydration process. Any improvement in the molecular sieve regeneration process that ultimately liberates capacity in the rectifier column may be advantageous. However, in many existing plants the beer column and rectifier columns are designed for equivalent capacities wherein the rectifier column is designed to have a predetermined fraction of capacity allocated for the ethanol-water. Increased capacity in the molecular sieve bed and/or rectifier would not eliminate the bottleneck of the beer column.
Needed is a process that allows free capacity in the rectifier column to be filled by alternative ethanol-water vapor and/or condensate streams from sources other than the existing beer column. Such a process would allow improvements in molecular sieve bed regeneration to be used in improving the overall throughput of existing and new plants.